(Na plus K ions) adenosine triphosphatase (ATPase) is the enzyme which is responsible for the coupled, active transport of sodium and potassium. At the present time, my laboratory is isolating both the cyanogen bromide fragments and the methionine-containing tryptic peptides of the large chain of (Na plus K ions) ATPase. Partial amino acid sequence information from these peptdes will allow us to order the cyanogen bromide framgents to form a linear map of this protein. This procedure will divide the primary structure of the large chain into 28 consecutive regions. We are also devising a two-dimensional gel eletrophoresis system in order to separate rapidly all of the fragments from a digest. In this way, it will be possible to determine in which of these 28 regions of the sequence a particular amino acid, which has been labeled covalently in some fashion is located. By utilizing specific amino acid modification reagents, it will be possible to determine which region of the linear sequence of the protein contain amino acids exposed to the external medium, the cytoplasm, and the liquid hydrocarbon interior of the bilayer. Each of the results of such experiments will position a region of the protein within one of these phases. As more and more information is gathered, it will be possible to coil the protein into a rough approximation of its three-dimensional structure. BIBLIOGRAPHIC REFERENCES: Kyte, J. (1976) J. Cell Biol. 68, 287-303. Kyte, J. (1976) J. Cell Biol. 68, 304-318.